Portable handheld electronic devices, such as the iPhone multifunction device by Apple Inc., have a touch screen in their front face, where an earpiece speaker or receiver used for telephony is located. When the device is being held against the user's ear during a phone call, a proximity function automatically senses this condition using an infrared proximity sensing device that is built into the device, and on that basis turns off the touch sensitive display screen of the device. The proximity function can also automatically determine or measure when the device has moved away from the user's ear, in which case the touch screen is re-activated. This is achieved by measuring the signals of an infrared proximity sensing devices radiation emitter and complimentary radiation detector, where the emitter transmits an infrared signal that is reflected by a nearby object (e.g., the user's head) and picked up by the detector. A stronger received signal may be interpreted by the proximity function to mean that the object is closer, while a weaker received signal means the object is farther away.
There are two primary near-field optical effects that have an impact on performance of the proximity sensing device. The first effect is the proximity sensing device's response to near-field low-reflectivity targets, such as a dark target object. A dark target object is one that tends to absorb a greater amount of radiation than lighter objects, therefore the intensity of a return radiation ray reflected off of a dark target object may not accurately reflect the dark target object's location or proximity to the proximity sensing device. The second effect is the device's response when liquid impurities are deposited on the surface of the touch screen interface directly above the proximity sensing device, such as oil-based secretions from the user's skin. This is collectively referred to as ‘smudge response.’ In some cases, the touch screen interface may have a coating such as an oleophobic coating, which facilitates cleaning of these human secretions (i.e., smudge) off of the screen. This coating, however, can also cause the smudge to bead up and form flattened spheres. These flattened spheres can act as optical lenses or total internal reflecting cavities for infrared beams emitted by the radiation emitter. In such cases, the infrared beams are reflected back to the radiation detector without contacting any nearby object, providing a false indication of user presence.
Each of these competing near-field optical effects must, therefore, be balanced to maintain proper proximity sensing device operation. Since both are near-field optical effects, however, changes to the optical/geometrical design of the proximity sensing device, or the electronic device within which it is implemented, typically result in an increase (or reduction) of both effects on the same order of magnitude. Thus, if a sensitivity of the proximity sensing device to, for example, a dark target object is increased, the sensitivity to smudge response is also increased. An increase in smudge response, however, is not desirable.